A Dolomite Ventifact

A ventifact is a stone that has been shaped by the erosive action of windblown sand (rarely ice crystals), bouncing along (saltating) only a few feet above the ground. The abrading grains of sand “file” (pit, etch, striate, groove, rill, and/or polish) away at the rock surface, which acts as a windbreak, and as result a sharp edge (facet) forms. Ventifacts form in arid environments (deserts) where there is a strong wind and a steady (but not overpowering) supply of sand or ice crystals. Windblow processes are referred also to as aeolian processes.  

Ventifact rock type (lithology) is commonly a very hard material like quartzite, chert, obsidian, or some basalts. Less common is dolomite, whose chemical formula is CaMg(C0₃)_2. Dolomite reacts only weakly with cold, dilute hydrochloric acid (HCl). Limestone (CaCO₃), which is another carbonate rock, is much softer than dolomite and, unlike dolomite, reacts vigorously with cold, dilute hydrochloric acid.

The ventifact shown below is a stone of dolomite (22 cm width and weighing about 2.5 pounds). Where it was found is unknown, but it was probably found in a desert in southern California.

The first view shown below is the top view of the stone. Most of the grooves are on the left side, but, if you look closely, you can see a smaller set of grooves on the lower right-hand area. Although this smaller set is in parallel alignment with the other set, the smaller set is at a sharp angle relative to the larger set. The only way to explain this phenomenon is that there had to been a change, over time, of 180° in wind direction. The facet (a sharp edge of demarcation between the two different-angled sides of the upper surface of the rock) is readily visible. 

For your information, the planet Mars has been modified by aeolian processes more than another other solid planet in our Solar System. One half of the rocks found at the Mars Path Finder Vallis Landing site in 1996 are ventifacts, and they were formed by northeast to southwest winds. For more information about that site, see the following article (free for reading):

Bridges, N.T., et al. 1999. Ventifacts at the Pathfinder landing site. Journal of Geophysical Research v. 104, no. E4, pp. 8595–8615.